apt-get clean: obsolete reference in Best Practices

[andrechalella]

About the important https://docs.docker.com/build/building/best-practices/.

The excerpt below links to a file that doesn't exist anymore. It has been removed for being unused and unmaintained in 2020 (along with its entire directory I guess), commit moby/moby@dc72890.

docs/content/manuals/build/building/best-practices.md

Line 440 in 9623874

Official Debian and Ubuntu images [automatically run `apt-get clean`](https://github.com/moby/moby/blob/03e2923e42446dbb830c654d0eec323a0b4ef02a/contrib/mkimage/debootstrap#L82-L105), so explicit invocation is not required.

This other file makes a similar claim to the above, only without links.

docs/content/contribute/components/code-blocks.md

Lines 213 to 214 in 9623874

	# Note: The official Debian and Ubuntu images automatically ``apt-get clean``
	# after each ``apt-get``

I think someone needs to check if the comment still applies and update the docs accordingly.